Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J35/00—X-ray tubes
  • H01J35/02—Details
  • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
  • H01J35/06—Cathodes
  • H01J35/065—Field emission, photo emission or secondary emission cathodes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y99/00—Subject matter not provided for in other groups of this subclass
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2201/00—Electrodes common to discharge tubes
  • H01J2201/30—Cold cathodes
  • H01J2201/304—Field emission cathodes
  • H01J2201/30446—Field emission cathodes characterised by the emitter material
  • H01J2201/30453—Carbon types
  • H01J2201/30469—Carbon nanotubes (CNTs)
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S977/00—Nanotechnology
  • Y10S977/902—Specified use of nanostructure
  • Y10S977/932—Specified use of nanostructure for electronic or optoelectronic application
  • Y10S977/939—Electron emitter, e.g. spindt emitter tip coated with nanoparticles

Definitions

• the present invention relates to an X-ray tube, to a medical X-ray device comprising such X-ray tube and to a method of operating such X-ray tube.
• X-ray radiography equipment may be used for various medical, analytical or other applications.
• an X-ray tube may be used to emit X-rays for transmission through an object to be analyzed, wherein the transmitted X-rays are subsequently detected and characteristics of the analyzed object may be derived from the detected X-ray absorption.
• a high current combined with a small focal spot of an electron beam may be desired for high spatial resolution.
• high temporal resolution may be desired which, inter alia, may depend on a switching time of an X-ray source used for acquiring the images.
• electrons are emitted from a cathode serving as an electron emitter and are accelerated by an electrical field towards an anode.
• hot cathodes are used for thermionic electron emission, wherein the cathode is heated up to very elevated temperatures such that the energy of electrons in the cathode may exceed the work function of the material used for the cathode such that electrons may escape from the surface of the hot cathode and the freed electrons may then be accelerated towards an anode.
• Electron emitters using the field emission effect seem to meet the above spatial and temporal resolution requirements and have the potential to be an ideal electron source for next generation X-ray tubes.
• WO 2010/131209 Al describes an X-ray source with a plurality of electron emitters using field emission.
• field emission of electrons may depend on a variety of parameters which may result in non-stable electron emission.
• an X-ray tube which comprises an electron emitter, a field generator and a heater arrangement.
• the electron emitter comprises a substrate with an electron emission surface. This surface has a roughness which is adapted for field emission of electrons from this surface upon application of an electrical field.
• the field generator is adapted for generating an electrical field adjacent to the electron emission surface of the electron emitter for inducing field emission of electrons from the electron emission surface.
• the heater arrangement is adapted for heating the electron emission surface contemporaneous with the field emission of electrons.
• a method of operating an X-ray tube as defined above with respect to the first aspect comprises generating an electrical field adjacent to the electron emission surface for inducing field emission therefrom and, preferably simultaneously therewith, supplying energy to the heater arrangement for heating the electron emission surface.
• the energy may be supplied to the heater arrangement prior to the generation of the electrical field for preconditioning the electron emission surface.
• the electron emission surface of the electron emitter may comprise carbon nano-tubes (CNT).
• CNT carbon nano-tubes
• Such carbon nano-tubes may be coated onto a surface of the electron emitter substrate and may provide for an electron emission surface having a high roughness as the carbon nano-tubes may have a diameter of only a few nanometers but a length which is much longer such that a plurality of nano-tubes may protrude from the electron emission surface like needles thereby supporting electron emission due to a field effect.
• the carbon nano-tubes may be coated directly onto a surface of the electron emitter substrate. No intermediate layer and/or binder may be used for attaching the carbon nano-tubes to the electron emitter substrate's surface.
• the electron emission surface may be heated to an elevated temperature of more than 100°C but less than an upper temperature limit at which the thermionic electron emission becomes greater than 10 % of the total electron emission or greater than 10 % of the field induced electron emission.
• the heater arrangement may be adapted for heating the electron emission surface to a temperature of between 100 and 1000 degree Celsius (°C), preferably between 200 and 900°C. Heating the electron emission surface to such elevated temperatures of well above ambient temperature but preferable well below a temperature where substantial thermionic electron emission occurs has been observed to provide for stable electron emission characteristics when the field effect is used for electron emission.
• the heating of the electron emission surface should be significantly below a temperature at which substantial thermal electron emission occurs as the heating only further optimizes the field emission.
• the elevated temperature to which the electron emission surface is heated should remain below a temperature where the thermionic emission from the electron emission surface or the CNTs is significant. Preferably, such thermionic emission remains below 10% of the total emission.
• the heater arrangement may be any arrangement adapted for directly or indirectly heating the electron emission surface of the electron emitter substrate. Any type of heating mechanism may be applied. For example, radiation heating using e.g. an infrared light source or a laser may be used for heating the electron emission surface. Alternatively, heat transport through a medium such as e.g. a channel or medium carrying heated liquid may be applied.
• the heater arrangement may use Joule heating, sometimes also referred to as resistive heating.
• the heater arrangement may comprise a resistive element arranged at the electron emitter substrate for heating the electron emission surface upon application of an electrical current to the resistive element.
• a heater arrangement using Joule heating by arranging e. g. an electrically resistive element in thermal contact with the electron emission surface may allow for a simple option for heating this surface to elevated temperatures.
• the X-ray tube may comprise a heater arrangement control which may be adapted for controlling an energy supply to the heater arrangement of the electron emitter for heating the electron emission surface to a predefined temperature.
• the heater arrangement may comprise a sensor for measuring the actual temperature of the electron emission surface such that based on such information the heater arrangement may be controlled to heat and hold the electron emission surface within a predetermined temperature range of e. g. in an average temperature +/- an acceptable temperature deviation of e. g. 50°C. Keeping the temperature of the electron emission surface in such predefined temperature range may help stabilizing electron emission characteristics.
• the heater arrangement control may be adapted for controlling an electrical current supplied to a resistive element provided at the electron emitter substrate for heating the electron emission surface. Such supplying of an electrical current may be easily controlled thereby obtaining a stabilized elevated temperature of the electron emission surface.
• the field generator of the proposed X-ray tube may comprise an electrically conductive grid. This grid may be arranged adjacent to the electron emission surface.
• the field generator may comprise electrical connections to the electron emission surface and to the grid such that a voltage generated in the field generator may be applied to these components thereby generating an electrical field between the electron emission surface and the grid. Due to such electrical field, electrons may be released from sharp tips comprised in the rough electron emission surface due to the field effect.
• the grid may furthermore be adapted such that these released electrons emitted from the electron emission surface may be transmitted through the grid towards an anode of the X-ray tube .
• a medical X-ray device comprising an embodiment of the proposed X-ray tube may be any type of X-ray radiography equipment, for example a computer tomography (CT) device.
• CT computer tomography
• Fig 1 shows an X-ray tube according to an embodiment of the present invention.
• Figure 1 shows an embodiment of an X-ray tube 1 according to an embodiment of the present invention.
• an electron emitter 3 and a rotating anode 29 are arranged.
• the electron emitter 3 comprises an electron emitter substrate 4.
• an electron emission surface 5 is provided by coating this surface with a multiplicity of carbon nano-tubes 19.
• Carbon nano-tubes are allotropes of carbon, typically with a cylindrical nano-structure.
• the length of the nano-tubes may be significantly larger than their diameters.
• the nano-tubes 19 are arranged on the electron emission surface 5 such as to produce a very rough surface in which at least some of the nano-tubes 19 protrude towards the anode 29 like thin needles. Tips of the nano-tubes 19 may serve as a source for emitting electrons due to field emission as at such tips an electrical field generated adjacent to the electron emission surface may be locally concentrated and may have locally elevated field strength. Due to such elevated field strength, electrons comprised in the nano-tubes may be released at such tips. Therein, the nano-tubes may have metallic or semi-conducting characteristics, depending on their specific properties like rolling angle and radius of the nano- tubes.
• the electrical field may be generated using an electrically conducting grid 9 arranged adjacent to the electron emission surface 5.
• a field generator control 23 comprised in a control 11 of the X-ray tube 1 may be electrically connected to both the electron emission surface 5 and the grid 11 such that a voltage of e.g. 2kV may be applied between these components.
• the resulting electric field may have sufficient strength for releasing electrons from the nano-tube's tips due to field emission.
• Electrons released from the electron emission surface 5 and forming an electron beam 35 may then be focused by an electron optics arrangement 21 controlled by an electron optics arrangement control 23 and may impinge onto the rotating anode 29 at a focal point 39.
• an X-ray beam 37 is generated as Bremsstrahlung. This X-ray beam 37 can exit the housing 31 through an X-ray transparent window 33.
• contaminations or adsorbents to the carbon nano-tubes may alter their electrical and/or geometrical properties thereby also altering electron emission characteristics.
• an organic binder has frequently been used for binding the carbon nano-tubes to a surface of a substrate.
• organic binder may outgas in the vacuum conditions within the X-ray tube 1 which outgasing may be detrimental to the vacuum and/or the electron emission characteristics.
• heating the carbon nano-tubes of the electron emission surface 5 to elevated temperatures well above the temperatures typically occurring in field emission emitters of conventional X-ray tubes may stabilize the electron emission characteristics of the electron emitter. Such heating procedure may be performed
• the heating procedure may precede the normal electron emission operation of the electron emitter 3 and may serve for preconditioning the X-ray tube 1.
• the heating of the electron emission surface 5 may be performed such that temperatures of between 200 and 900°C, preferably between 400°C and 900°C, are attained at the electron emission surface 5. Such temperatures are well above the ambient temperature or the temperature at which the electron emitter 3 would be without any additional heating. On the other side, the upper limit of the temperature range is well below typical temperatures used in thermionic emitters. In other words, while additional kinetic energy may be provided to electrons comprised in the carbon nano-tubes of the electron emission surface due to the elevated temperature, an upper limit for the temperature may be chosen such that this additional energy is still well below the work function energy of the material of the electron emission surface, i. e. for example of the carbon nano-tubes, such that no substantial flow of released electrons occurs due to thermionic emission.
• the electron emitter 3 operates as a field effect electron emitter such that a flow of released electrons may be controlled by controlling the electrical field generated between the grid 9 and the electron emission surface 5.
• an electron beam emitted towards the anode 29 may be varied and may for example be switched ON and OFF, thereby also enabling varying of the X- ray beam 37.
• a heater arrangement 15 is provided for the X-ray tube 1. While, in general, any heater arrangement enabling heating the electron emission surface 5 to the required elevated temperatures may be used, a specific type of heater arrangement 15 shall be described in the following in more detail. However, it shall be noted that other types of direct or indirect heater arrangements relying for example on resistive heating, radiation heating, conduction heating, induction heating or similar may be used.
• a resistive element 17 is comprised in the substrate 4 of the electron emitter 3. Such resistive element 17 may form a part of the substrate 4 or may form the entire substrate 4.
• the resistive element may have an electrical resistance such that upon applying an electrical voltage and thereby inducing an electrical current, Joule heat is generated within the resistive element 17 and is transferred to the electron emission surface 5.
• the resistive element 17 may be electrically connected via lines with an energy source of the heater arrangement control 23 for controUably supplying electrical energy to the resistive element 17.
• the heater arrangement control 23 may be adapted for controlling an electrical current supplied to the resistive element 17 such that the electron emission surface 5 is heated to a temperature within a predefined temperature range, for example to a temperature of 850°C +/- 50°C. Keeping the temperature of the electron emission surface 5 in such a temperature range may for example prevent contamination of the carbon nano-tubes of the electron emission surface 5 and may furthermore lower the work function necessary for releasing electrons from the carbon nano-tubes due to the field effect. As a result, the emission of electrons from the electron emission surface 5 may be stabilized.
• the heater arrangement control 23 may be part of a general control 11 of the X-ray tube 1 comprised externally or internally within the X-ray tube 1 and further comprising a field generator control 25 for controlling the electrical voltage applied to the electrodes of the field generator 7 and further comprising an electron optics control 27 for controlling the electron optics 21.

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• X-Ray Techniques (AREA)

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• 2012
  • 2012-11-14 EP EP12812366.8A patent/EP2748834A1/de not_active Withdrawn
  • 2012-11-14 IN IN3833CHN2014 patent/IN2014CN03833A/en unknown
  • 2012-11-14 CN CN201280058403.0A patent/CN103959422A/zh active Pending
  • 2012-11-14 US US14/360,661 patent/US20140321619A1/en not_active Abandoned
  • 2012-11-14 BR BR112014012484A patent/BR112014012484A2/pt not_active Application Discontinuation
  • 2012-11-14 WO PCT/IB2012/056417 patent/WO2013080074A1/en active Application Filing
  • 2012-11-14 RU RU2014126428A patent/RU2014126428A/ru not_active Application Discontinuation
  • 2012-11-14 JP JP2014542965A patent/JP2015504583A/ja active Pending

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